I was playing a 4 person game taoda and on a couple of occasions I was not allowed to place trains on, what appears to be, valid spaces. The last occasion was on two vacant yellow spaces between Denver and Santa Fe when I had 3 yellow tarins available. The annoying thing was that this prevented me from completinga Route.

In the board game, you get 45 actual little train carriages, when you run out, you can't play any more. Same story in the digital version. Usually, if you have the right number of cards and they're the same colour, and you still can't claim a route, you probably didnt have enough train carriages left. Check that down the bottom of the screen, near your score.